1. Field of the Invention
The present invention generally relates to a semiconductor package, and more particularly to a wire bonding structure of a semiconductor package, wherein a middle material is located between a copper bonding wire and an aluminum pad so as to be acted as a buffer during the pressing process, whereby the force resulted from the copper bonding wire cannot damage the structure of the aluminum pad during the pressing process.
2. Description of the Related Art
Referring to FIG. 1, according to a process for manufacturing a semiconductor package, the technology of wire bonding method widely applied to the electrical connection between a pad 11 of a chip 10 and a pad 13 of a substrate 12 by using a bonding wire 14. Wire binding process is mainly based on gold (Au) wires, but copper (Cu) wire has an advantage of low cost. Compared with the gold, the copper has better electric conductivity and thermal conductivity, whereby the copper bonding wire has smaller diameter of wire and better dissipation of heat. However, the copper has disadvantage of insufficient ductility and easy oxidation such that the utilization of the copper bonding wire is restricted.
Recently, the copper bonding wire is only applied to a chip pad with a big size or a chip pad of low dielectric material (low-k) of wafer, because the success of the wire bonding process of the copper bonding wire depends on the structural strength of the chip pad. In order to avoid the failure of the wire bonding process of the copper bonding wire, the small size of the chip pad will be limited.
Referring to FIGS. 2 to 4, they depict a conventional method for bonding a copper bonding wire. Referring to FIG. 2, a copper bonding wire 20 is provided by a wire bonding machine, wherein the copper bonding wire 20 has a copper line 22 and a copper ball 24. The copper ball 24 is physically connected to an end of the copper line 22 by a discharging electricity manner or a burning hydrogen manner. Referring to FIG. 3, the copper ball 24 is pressed and then deformed. Referring to FIG. 4, the deformed copper ball 24 is bonded to an aluminum (Al) pad 32 by a vibration process. However, the hardness of the copper is higher than that of the aluminum, and thus the force resulted from the copper bonding wire 20 possibly damages the structure of the aluminum pad 32 during the pressing process. Furthermore, an inter-metallic compound (IMC) is formed between the aluminum pad 32 and the copper bonding wire 20, and the quantity of IMC is insufficient. Thus, the wire bonding structure in the prior art has less bonding force and further has lower reliability.
Referring to FIG. 5, U.S. Pat. No. 6,329,722 B1, entitled “Bonding Pads for Integrated Circuits Having Copper Interconnect Metallization”, discloses that a device having a thin metallic coating 70 (e.g. tin) which forms strong bonds to copper is provided on the pads 60 of an integrated circuit having copper metallization. The surface oxidation of the thin metallic coating 70 is self limited, and the oxides are readily removed. Furthermore, the pad 60 having the thin metallic coating 70 forms intermetallics at low temperature, thereby making the pad 60 both solderable and compatible with a bonding wire 80.
Although the tin metallic coating can form intermetallics at low temperatures, it is necessary to consider the oxidation problem of the pad made of copper in the prior art, e.g. the tin metallic coating must cover the whole exposed area of the copper pad so as to avoid the oxidation of the copper pad.
Accordingly, there exists a need for a wire bonding structure of a semiconductor package capable of solving the above-mentioned problems.